#define _CRT_SECURE_NO_WARNINGS 1
#include<stdio.h>
#include<math.h>
int main()
{
    int i = 1;
    double eps, n = 1, m = 1, t = 1, sum = 1;
    scanf("%le", &eps);
    while (n > eps)
    {
        m = m * i;
        t = t * (2 * i + 1);
        n = m / t;
        sum = sum + n;
        i++;
    }
    printf("PI = %.5lf", sum * 2);
    return 0;
}
